Particular fan units often referred to as tubular fans, are directly fitted in ducting for air or air/gas mixtures which are axially drawn in and axially blown longitudinally through the ducting.
It is common that, due to expense, axial fans for moving air or other media are used in conjunction with a distributor, diffusor or guide device on the output end for directing the emerging air. These fans, however, have disadvantages due to relatively poor efficiency, high noise level, and allowing the air to emerge with a spin. Because the spin energy transferred to the air by the impeller is not able to be converted back into pressure which can be utilized and, consequently, remains in the form of rotational energy in the output flow. A waste of power and a high noise level results. Furthermore, the spin still present, in the fluid, such as air being moved, produces low frequency sound vibrations of the ducting and secondary sound emission is transferred to the further downstream fixtures fitted in the ducting.
Suggestions have already been made to use radial fans in the ducting, in the case of which the impeller is placed in a radially symmetrical housing with coaxially placed inlet and outlet connectors. The air emerging radially from the impeller is redirected back in an axial direction to the outlet connector. The air is brought together with the aid of an air outlet cone narrowing towards the outlet connector. Such fans combine the advantages of an axial type of fan; that is to say straight through flow, with those of a radial fan; namely, a high pressure and a relatively low level of noise. These fans have become very popular for some applications. Their assembly is simple, just as simple for example as the fitting of a length of ducting or tubing, the air outlet being aligned in direction with the air inlet, that is to say axially.
Known arrangements of the type in question here do, however, have the disadvantage that the air emerges with a substantial spin from the impeller so that the volumetric flow continues to move helically even in the next part of the ducting or tubing system and this leads to a substantial loss of power. A large proportion of the energy imparted to the fluid is not able to be utilized so that the efficiency of such fans is correspondingly poor. Accordingly, an attempt was made (see the German unexamined specification No. 3,439,780) to arrange guide vanes in the outlet cone in order to contribute to the recovery of spin energy into pressure which might be utilized. However, the guide vanes forming a guide grid in the cone did not produce sufficient effects and furthermore the manufacture of the housing and the outlet cone is not economical. A further proposal made in U.S. Pat. No. 3,650,633 involved the provision of a cylindrical housing in order to simplify manufacture. A disadvantage with this system is that there has to be an internal cylinder coaxially placed downstream from the impeller and forming the hub of the impeller. Guide vanes are arranged between the internal cylinder and the outer housing. In this arrangement, the emerging air is substantially free of spin but the flow pattern has an annular cross section so that in the following duct it is necessary for the air to be distributed over the full cross section. This involves a sudden increase in the cross section which causes relatively high losses.
Finally, a proposal has been made in the German unexamined specification No. 1,628,335 to design a fan unit with a freely rotating radial impeller without a spiral housing and which was operably connected to a drive motor and acted on the gaseous fluid in an axial direction and expelled it in a radial direction. A guiding means or distributor was set thereon on the inner side of the external fan housing and had a plurality of guide vanes extending in the axial direction to deflect the fluid flow emerging from the radial impeller in an axial direction. This previously proposed device thus has the features of a fan as specified initially herein but however it was so designed that the laminar flow of the fluid originally produced at the point of deflection was converted into a turbulent fluid flow owing to the form of the annular gap through which it had to pass and owing to the conical form of the housing. A further disadvantage of the design was that the effective area decreases at the point of deflection toward the inlet port and this also tends to lead to turbulence. The flow of fluid has to overcome a relatively high resistance to flow and there is a danger of repeated turbulence producing effects acting on the flow. Consequently, the flow transition between the radial impeller and the distributor needs substantial improvement. On incorporation of this known system in piping or ducting there may be difficulties as regards attachment of the housing within the tubes and furthermore manufacturing of the fan unit is relatively involved owing to the configuration of the vanes and to the structure of the housing.